A fuel cell includes a fuel cell stack made of a lamination body of a plurality of fuel cells (i.e., a plurality of single cells). Each of the plurality of single cells has a membrane-electrode assembly including an anode electrode on one surface of an electrolyte membrane and a cathode electrode on the other surface of the electrolyte membrane. The membrane-electrode assembly is sandwiched between a gas flow channel layer and a separator. The anode electrode is supplied with fuel gas containing hydrogen. The hydrogen is subjected to an oxidation reaction according to the following chemical formula (1) to thereby generate proton from the fuel gas. Thus generated proton travels to the cathode electrode through the electrolyte membrane. The cathode electrode is supplied with oxidant gas containing oxygen. The oxygen in the oxidant gas reacts with the proton travelled from the anode electrode to generate water according to a reductive reaction expressed by the following chemical formula (2). As a result, an electrogenic reaction according to the following chemical formula (3) occurs throughout the fuel cell. The fuel cell extracts electric energy from electrodes through an electrochemical reaction occurring on a surface of a side of the electrolyte membrane of the paired electrode structure body.H2→2H++2e−  (1)(½)O2+2H++2e−→H2O   (2)H2+(½)O2→H2O   (3)
As described above, since the fuel cell generates water (Thus generated water is indicated at 30 in FIG. 1.), the generated water within the fuel cell freezes in a cold environment, e.g., below freezing temperature, whereas, the once frozen generated water is defrosted by heat generated at drive of the fuel cell and returns to water again, resulting in causing a possible building-up of water within the fuel cell. If the freezing and building-up of water occurs within the fuel cell, the reactant gas flow channel is blocked to disturb gas diffusion, thereby causing drop in output of the fuel cell. In the light of the above described problem, Japanese Patent Application Publication No. 2005-44795 discusses a method for improving power generation characteristics by controlling pressure of a reactant gas supplied to a fuel cell stack upon actuation thereof below freezing temperature to a value slightly higher than pressure upon normal driving. Increase of a supply pressure of the reactant gas forcibly supplies gas to a reaction surface to compensate decrease of the gas diffusion.